BUT there should be a way to make a unified extension for all operating systems. In fact ssitter had written an article to do that. It needs some updating to account for the Mozilla 2 XPCOM changes, however.
The steps are mostly the same, with some added.
- Download whatever versions you want (perhaps 1.0b4?).
- Unzip them all and choose which one will be your "unified" version.
- Find the <em:targetPlatform> section in the install.rdfs and include all the necessary ones.
- Find the libcalbasecomps.* (where * is dll, dylib, so, etc. for your platform) inside the components directories.
- Copy the libraries together into one directory.
- Copy the lines from each components.manifest together into one manifest.
- For some systems you'll need to specify the ABI (and possibly change the path), note that the ABI is just what was inside the <em:targetPlatform>.
- Rezip this up into an XPI and you should be good to go!
Edit: I should also mention that you would need to do something similar with the themes folders if they differ dramatically (it seems there's just two: winstripe and pinstripe: i.e. Windows & Mac; Linux and Solaris most likely also use winstripe).